This invention relates generally to controls for moisture feed systems in lithographic duplicators and, more particularly, relates to systems in which the moisture carrying form roll also attracts ink.
It is conventional in the construction of the moisture feed systems for lithographic duplicators to use a moisture form roll that is ink receptive. The moisture form roll can therefore carry ink as well as apply moisture. Such systems have been referred to as "bare-back systems" for the reason they obviate the need for the well known molleton covered form roll. Molleton covers, as is well known in this art, are made of cloth which soaks up moisture and serves as a reservoir to apply moisture to the master.
In the bare-back system, the master has moisture applied from the special moisture form roll to its water-receptive areas.
In the start-up operation of a bare-back moisture feed system, the moisture form roller receives a layer of ink usually by permitting ink to be applied to the metal surface of the master cylinder. The cylinder is then cleaned and a fresh moistened master then receives ink application from the ink form roller. The duplicating process of running the master through the cycle on the machine causes a certain amount of ink to be exchanged between the image portions of the master and the ink layer of the moisture form roller.
The layer of ink which ultimately accumulates on the moisture form roller appears to promote uniformity in its surface characteristics and thereby improve its properties as a moisture forwarding agent. It will be understood that the layer of ink on the moisture form roller is important to its operation in this connection and, therefore, the character of the ink layer itself is important.
It has been found that the oleophilic character of the moisture form roller provides the necessary environment to control the moisture supply by means of a frictionally responsive moisture transfer roll. The moisture transfer roll acts as a valve between the moisture supply and the moisture applying element, namely, the moisture form roller. The oleophilic moisture form roll is made to run in rolling contact with the moisture transfer roll whose surface is primarily hydrophilic, and which is driven solely by a nominal or non-film control pressure contact between its surface and that of the ink covered moisture form roll via the clear resistance offered by the intervening layers of the ink and the moisture. The detailed description of the valve control function carried out by the moisture transfer roll in the bare-back type moisture system of a lithographic duplicator is described in detail in co-pending applications Ser. Nos. 682,855 and 689,165, both filed in the name of S. A. Mabrouk and assigned to the same assignee as the instant invention. The aforementioned pending applications are concerned with the feeding of moisture to the transfer roll by a supply system which provides a proper moisture flow allowing the transfer roll to accept from the system the amount of moisture which is currently required as a function of its speed of rotation and which speed of rotation is governed by a mechanism applying a restraining torque to the rotation of the transfer roll. The mechanism is characterized by exerting a restraining effect which increases monotonically as the speed of the transfer roll rotation increases.
Hence, it will be seen that the character and quality of the ink layer on the moisture form roller is important, since as explained earlier, the moisture transfer roller is driven solely by the shear resistance offered by the intervening layers of ink and moisture.